Method of making consolidated lignocellulose boards



Aug. 28, 1962 J. G. MEILER 3,050,782

METHOD OF MAKING CONSOLIDATED LIGNOCELLULOSE BOARDS Filed March 5, 1959 2 Sheets-Sheet 1 PRESS/N6 V ATTORNEYS Aug. 28, 1962 J. G. MEILER 3,050,782

METHOD OF MAKING CONSOLIDATED LIGNOCELLULOSE BOARDS Filed March 3, 1959 2 Sheets-Sheet 2 l I I LL 500 L(5TEAM PRESSURE AT 465F) n: ---r--- a 1 m 400 LIJ [If n I l l 0 /0 60 I00 //0 I20 I30 TIME IN SECONDS INVENT OR United States Patent Filed Mar. 3, 1959, Ser. No. 796,770 6 Claims. (will. 18- 375) This invention relates to the production of boards from lignocellulose particles and, more particularly, to the consolidation by heat and pressure of a felted mat of such particles into the finished board.

Many difierent methods have heretofore been developed for the production of board products by reducing wood or other lignocellulose products to fine particles, forming such particles into a unitary mat and consolidating the mat by the application of heat and pressure into a solid board. In general, such methods may be classified into three categories; namely, the wet, wet-dry, and dry processes.

In the wet process, a pulp mat is prepared from an aqueous slurry of wood fibers in the manner similar to the conventional paper manufacturing process and pressed to produce a fiberboard panel or hardboard. Among the disadvantages of the wet process are the decrease in yield due to the loss of essentially all of the water soluble components initially in the wood and the relatively high percentage of such materials produced by the more drastic cooking conditions necessary in the wet process; the nonuniformity of the mat due to clot formation and orientation of the fiber in the direction of the water flow during mat formation; the expense of supplying and handling the slur and the serious problem of disposal of a large volume of objectionable efiluent.

In the wet-dry process, a wet mat is formed in the same general manner as in the wet process; but, instead of being pressed while wet, the mat is dried prior to final pressing to produce a product which is initially a low density board such as the common insulation board. This low density board is subsequently subjected while dry to heat and pressure to increase its density and produce a rigid board. In the wet-dry process, not only is the operation expensive and the water solubles lost as in the wet process, but the pressing of a dry insulation board produces an almost black high density board which is much less desirable than the lighter colored, stronger board produced by the dry process.

Use of the dry process has become increasingly more widespread in recent years. The term dry process indicates that the fibers are conveyed to the mat-forming operation in a gaseous rather than a liquid vehicle. Although an aqueous slurry is never formed, the fibers contain some moisture.

One problem which has confronted practitioners of the dry process has been the production of hardboard having a hard, smooth surface with a fine, non-fibrous texture. Such surfaces cannot be produced in the absence of some moisture in the mat when it is cured. Apparently, the desired surface characteristics accrue, at least in large part, from the plasticizing effect of the water. The amount of water present in the mat must be maintained below a given maximum because higher moisture contents produce flow spots, surface blemishes and blisters in the finished board. The removal of the water as steam causes blisters, flow patterns and otherwise disrupts the uniformity of the board surface. Various methods, none of which are entirely satisfactory, have been employed by the prior art to effect the necessary hot pressing to the desired density and to control the escapeof steam to the end that hard, smooth, finetextured surfaces result.

Ce Patented Aug, 28, 1962 In one method frequently used heretofore, the moist mat is hot pressed between a smooth caul plate on one face of the mat and a foraminous member such as screen on the other face. In this method, most of the steam escapes through the foraminous member and damage to the surface adjacent the smooth caul plate is minimized. One obvious disadvantage of this method is that only one smooth surface may be formed on the resulting product.

The prior art, in attempting to produce hardboard having smooth surfaces on both sides, has resorted to a number of generally costly and complex pressing techniques. In certainprior art processes, the moist mat is first toasted at low pressure between hot caul plates before the high pressure necessary to produce the desired density is applied. Such toasting is not only time-consuming but releases much of the moisture in the mat, the pla-sticizing efiect of which is thereby sacrificed in the high pressure phase of the pressing operation. Excessive moisture loss results in the formation of a soft fibrous surface on the product.

In other prior art processes, the mat faces are sprayed with additional quantities of water before being pressed between a pair of caul plates. This procedure is expensive and generally requires an exceedingly complex timepressure cycle to release the moisture without damaging the board surfaces.

In addition to the hereinbefore delineated surface characteristics, there is an increasing demand for hardboard with high resistance to expansion, particularly longitudinal expansion, due to moisture absorption. This dual requirement has not been adequately met by heretofore known processes.

To overcome the disadvantages of the prior art, a

' primary object of the invention is to provide an improved method of consolidating lignocellulose particles into a board characterized by smooth, hard, fine-textured surfaces on both sides and high resistance to expansion due to moisture absorption.

A further object of the invention is the consolidation of a slightly moist mat of lignocellulose particles into a dense, smooth-surfaced board by an improved heat-pressure cycle characterized by greater economy and faster operation than heretofore known processes for consolidating such slightly moist mats.

Still another object of the invention is to provide an improved method of producing hardboard by controlled application of heat and varying pressure to a slightly moist mat of wood fibers to produce a board having smooth, hard, blister-free surfaces on both sides and improved dimensional stability.

The invention broadly embraces a method of producing a board which comprises forming lignocellulose particles into a compact mat containing from about 6% to about 16% of water and from about 0.5% to about 10% of a resinous binder, said percentages being based on the dry weight of said particles, positioning said mat between a pair of smooth, imperforate surfaces, rapidly subjecting the faces of said mat to a temperature of from about 380 F. to about 520 F. and to an inital high pressure greater than the pressure of saturated steam at the operating temperature and in the range of from about 600 p.s.i. to about 1200 p.s.i., maintaining said initial high pressure until the mat is compressed to a desired density, releasing said initial high pressure at a relatively rapid rate to an intermediate pressure substantially below the pressure of saturated steam at the operating temperature but not less than p.s.i. to permit the escape of steam from the mat with an audible squeal, maintaining said intermediate pressure ,until said squealing substantially ceases, and thereafter reducing the pressure applied to said mat to zero to produce a board characterized by specific examples thereof and water ratios may be used in each blender.

if uniform resin content throughout the mat: is desired, the blenders -may be eliminated and the resin may be mixed inthe refiner simultaneously with the defiberizing operation.

"-is in the'range' of from about 0.5% to about In addition a water repellent such as wax may be added in' a"ra'tio froin'about O.5% 'to about 4%-based on the dry weight of the fibers. Preferably the amount of wax and hard, smooth, abrasive-resistant surfaces substantially undamaged by escaping steam.

The invention having I V v now be set forth in greater detail with reference to the accompanying drawings in which: ,1

FIGURE 1 is a flow sheet showing a dry process for forming hardboard;

FIGURE 2 is a sectional view of a press which is employed to consolidate the mat in the'hot pressing step; and

'such as hickory, oak, beech, birch, and maple. Desirably, the wood particles are reduced to ultimate fibers and opened-up aggregates of ultimate fibers which, when formed into a .composite felt, have good interlocking characteristics.

In forming wood fibers, the logs are run through a conventional chipper as is common practice in the wood pulping industry. With reference to the flow sheet of FIGURE 1, the chips are stored in and fed from chip silos into a conventional steam cooker, such as a Grenco continuous cooker. Alternatively, other conventional or rapid cycle digesters may be used to perform the cooking operation. In the cooker, chips are steamed under pressure in the well-known manner for a time sufficient to soften the chips. a 7

From the cooker, the hot, softened chips are transferred to any conventional refiner suchas the Bauer refiner for reduction to fibers. The Bauer refiner, which is preferred, includes a pair of oppositely rotating grinding discs which 'are positioned facing one another at slightly spaced locations to grind the softened chips into particles comprising essentially ultimate fibers and opened-up aggregates of ultimate fibers, i.e., loosened collections of a few individual fibers.

The fibers are conveyed from the refiner by heated air and/or hot combustion gases and are thereby dried to a moisturecdntent of'from about 6% to about 16% and preferably from about 8% to about 12%. Before the fibers are felted into a mat, they are subjected to any desired combination of steps of air separation, classificatiornresin binder mixing, and the like, to meet special requirements. In one preferred combination of steps,

the fibers are conveyed to cyclones where a desired amount of air is removed; From the cyclones, the semidry fibers are transferred to a classifier for separation into fine'and coarse fiber components. The fine and coarse components may be conducted to separate blenders and mixed with resin binder. The fine fibers which ultimately will constitute the outer layer or layers of the hardb oard may receive a higher percentage 'of resin than the coarsefibers'which constitute the inner layers. Moreover, different types of resins and different resin Alternatively,

Preferably, the resin content of the fibers been generally described will l i which is added'is in the range of from about'1% to about3.5%.

The fibers having been mixed with resin binder and water repellent to the desired ratios, dried to the desired moisture content and classified into fine and coarse components are air conveyed to a felter. ing operation, the classified fibers are blown downwardly onto a moving foraminous belt to form a multiple/layer mat with coarse fibers in the middle layers and fine fibers in one or both of the outside layers. single layer mat may be formed in the felter in which case no classification of the fibers prior to felting is neces:

sary. The felted mat is pro-compacted to a substantially self-sustaining condition and is then ready for, the final curing operation in which the mat is thickness and the binder cured.

' 'It is essential that the average moisture content or. the

mat at the time "of final curing be in the range of-from 0 about 6% to about 16% of'the dry Weight of the fiber and preferably from about 8% to about 12% of such dry weight. When the average moisturecontent is too low, a soft, fibrous surface is produced on the 'hardboard by the pressing cycleof the invention. When the average moisture content is too high, the pressing cycle of the invention tends to produce blisters and flow marks on the board surface. Further, the mat retains its resin content of from about 0.5% to'10% and its wax content of from 0.5% to about 4% of the dry fiber weight.

With reference to FlGURE 2, the thus-constituted mat is'placed between a'pair of smooth, imperforate metal caul plates 1 which, in turn, are positioned in a standard hydraulic press 2. Throughout the pressing operation, the edges of the mat are exposed to the atmosphere. During the pressing operation, the caul plates are heated to a temperature of from about 380 F. to about 520 F. and preferably from about 400 F. to about 500 F. The caul plates are heated by the platens 3 and 4 of the press which in turn are heated by steam, electric heaters, or any other suitable heating means. It will be understood that the caul plates may be at a lower temperature when they are inserted between the hot platens of the press but are quickly heated by such platens to operating temperature. I

A typical time-pressure cycle according to the inyen to apply an initial high pressure on both facesof the mat between the caul plates. The time required to reach such highpressure will vary, depending upon the characteristics of the particular press used; but'it'can be reached in 20 seconds or less. Simultaneously, heat is transmitted to the mat through the caul plates. The rapid application of initial high pressure tobot facesof the that as described while the mat retains sub .stantially all of its moisture content enables the production' of a board having smooth, hard, fine-textured,-nonacteristics to the finished board and Will compress the mat to a' predetermined density without producing a" .fibrous surfaces .on both sides.

sure must be Within a rangew'hich, at the particular tem- The magnitude of presperature employed, will impart the desired surface charboard with blisters'or otherwise disrupted surfaces. The

initial high pressure must "be maintained for a period of time sufficient to compress the matto the desired density which approximates the density of the finalboard; and

usually is slightly greater than such final density as more] fully explained hereinafter. If .such pressure is mains tained too long, however,- not only willthe density of the board be too high, but the surface of the board may be blistere-d or otherwise disrupted. I 7

Depending upon the thickness and desired density of the board to be produced, the initial high pressure should "be withina range of from about 600 p.s.i. toabout 1200 In a typical felt- Alternatively, a

compressed to final p.s.i. applied for from about 1 second to about 45 seconds at a caul plate temperature of from about 380 F. to about 520 P. All presures referred to herein are gage pressures. Preferred ranges are from about 750 p.s.i. to about 1000 p.s.i. applied for from about 5 seconds to about 20 seconds at a caul plate temperature of from about 400 F. to about 500 F. Within the stated ranges, the initial high pressure should always be higher than the saturated steam pressure at the particular temperature employed to prevent appreciable escape of steam during the high pressure step.-

I'he selection of particular temperature, pressure, and time values within the stated ranges depends upon a number of factors. For example, as the thickness of the board to be produced increases, the time of the application of pressure must be increased to obtain a given density board. To produce a board with a given density and thickness, a relatively high pressure is applied for a relatively short time; and, when progressively lower pressures are employed, the application time is correspondingly increased. In general, it is preferable to use relatively high pressures for relatively short times rather than lower pressures for longer times. It will be apparent that many possible combinations of pressures, temperatures and times Within the stated ranges may be employed to achieve given results.

It was heretofore known that the application of high pressure in the presence of heat to a slightly moist mat tends to improve the surface characteristics of the resulting board, but the release of the moisture from within the mat after such application of high pressure in a manner which prevents blistering and otherwise disrupting the board surface has presented a problem which heretofore has not been satisfactorily solved. Thus, this invention resides, not in the initial high pressure step per se, but rather in an improved, complete time-pressuretemperature cycle which includes additional steps to finish the pressing operation and to effect controlled dissipation of the moisture within the mat to produce at a rapid rate boards which not only have excellent surface characteristics but also possess excellent strength and high resistance to moisture absorption and expansion due to moisture absorption.

After the mat has reached the desired density, the initial high pressure is reduced at a relatively rapid rate to an intermediate pressure within a range as specified hereinafter to permit controlled escape of most of the moisture within the mat as steam. Such reduction is effected in from about 2 to about 30 seconds, depending on the thickness of the board to be made. The intermediate pressure must be below the steam pressure within the mat at the particular operating temperature to permit the steam to escape from the mat with an audible squeal. in the specific example depicted in FIG- URE 3 where the temperature which is employed is 465 F., the intermediate pressure of 200 p.s.i. is substantially below the steam pressure at such temperature and as a result the steam escapes with an audible squeal. The intermediate pressure must not be so low, however, that the steam escapes sufficiently rapidly to cause disruption of the surfaces of the consolidated mat.

It has been found that the minimum intermediate pressure to which the mat may be subjected to obtain an excellent product is about 100 p.s.i. Even better results are obtained if the intermediate pressure is maintained above about 150 p.s.i. when operating above 400 F. The minimum intermediate pressure preferably is slightly higher in pressing a plural layer mat with fine fiber faces than in pressing a single layer mat and is higher at the higher operating temperatures than at the lower temperatures.

'The intermediate pressure is maintained until most of the steam has escaped and squealing substantially ceases. It is very important that the mat not .be vented until most of the steam has been permitted to escape as evidenced by the absence of squeal. Venting, or the release of pressure to below about p.s.i., not only causes surface disruption, but lowers the strength and resistance to moisture absorption of the final board. Specifically, such intermediate pressure is held for from 30 to 300 seconds in the production of A5 hardboard and 30 to 420 seconds in the production of hardboard. After the squealing ceases, the pressure is reduced to zero. The reduction of the intermediate pressure to zero must be sufiiciently gradual to prevent blistering or otherwise disrupting of the surface ofthe resulting board by the excessively rapid escape of the remaining steam from within the mat. Normally, however, very little steam remains in the mat after the squealing ceases and the reduction of the intermediate pressure to zero may be accomplished at a reasonably rapid rate.

In general, the upper limit of the intermediate pressure on the mat is the highest pressure at which steam escapes from the mat with an audible squeal. Such squeal occurs when the pressure on the mat is sufficiently below the steam pressure within the mat so that the escaping steam overcomes the resistance to flow presented by the fibers within the mat and the external pressure exerted on the mat by the caul plates. Assuming that, during the intermediate pressure step, all portions of the mat are at a temperature substantially equal to the temperature of the platens, the steam pressure within the mat theoretically would be the pressure of saturated steam corresponding to the platen temperature. It may be, however, that some portions of the mat are cooler than the platens during the intermediate pressure step and thus the actual steam pressure in the mat may be lower than the theoretical saturation pressure at the platen temperature.

Thus, for excellent results, the intermediate pressure on the mat should be substantially less than the theoretical pressure of saturated steam at the operating platen temperature to permit the escape of steam with an audible squeal. :Preferably, the pressure differential between such saturated steam pressure and the intermediate pressure on the mat should be at least 100 p.s.i. and even higher at the higher operating temperatures, where the saturated steam pressure is very high (about 800 p.s.i. at 520 F.). Best results are obtained when the intermediate pressure is in a range below about 500 p.s.i. which at the higher temperatures may be up to 300 p.s.i. below the saturated steam pressure.

One reason why a higher pressure difierential between the saturated steam pressure at the operating temperature and the intermediate pressure on the mat is preferred at the higher temperatures is that too high an intermediate pressure on the mat will result in a board with an undesirably high density. Generally, the highest density to which the mat is compressed occurs at the end of the initial high pressure step and is slightly higher than the final density of the board because the mat springs back when the pressure is reduced. However, where the initial high pressure is applied for a relatively short time and the intermediate pressure is relatively high, the highest density of the mat could be reached at the end of the intermediate pressure step. If the intermediate pressure is too high, it follows that the'density would be excessive. At high temperatures, the steam pressure within the mat is very high. If the intermediate pressure approaches these high steam pressures and is maintained for an extended period, the density of the mat would be increased to a marked degree. Accordingly, to minimize such density increase at high operating temperatures, the pressure differential between the pressure of saturated steam at such temperatures and the intermediate pressure on the mat should be relatively great.

For the foregoing reasons, the preferred intermediate pressure range is from -a minimum of about 100 p.s.i. to a maximum of not'in excess of about 100 p.s.i. below the pressure of saturated steam at the operating temeach of which contained 1.5% "resin binder.

'perature, but not more than 500 psi. to permit the escape of steam from the mat with an audible squeal. For any given operation temperature, the magnitude and maintenance time of both the initial high pressure and the intermediate pressure must be such that the finished board will have the desired density. Because the presence of water in the mat enables better surfaces to be formed, the initial high pressure should be applied as rapidly as possible, should be as high as possible, and should be maintained as long as possible consistent with the formation of a blister-free board having the desired density. The mat should be compressed to a density approximating the density of the final board andpreferably slightly higher than such final-density during the initial high pressure step.

Boards produced according to the process described herein have fine-textured, non-fibrous surfaces which are hard, smooth, abrasive-resistanuand free from blisters, flow marks and other blemishes. The strength of such boards is as great'or greater than boards produced by the complex methods of theprior'art. Moreover, the process of the invention produces-boards having an extremely high resistance to expansion and particularly longitudinal expansion due to moisture absorption. Another and important advantage of the process of the invention resides in the rapidity of the pressing cycle which permits a high rate of production.

The process is especially advantageous in the production of boards having a specific gravity of from about 0.8 to about 1.2, but boardsof greater or lesser density may be manufactured by the process.

Certain advantages of the process of the invention are apparent from the following examples:

about to seconds were required to close the, press to reach the initial high pressure. The time and magnitude of the initial high pressure isgiven, in the first column under the heading Time-Pressure Cycle. Another 2 to secondswere required to reduce the initial high pressure to the intermediate pressure. The time and magnitude of the intermediate pressure is given in the second column under the heading Time Pressure Cycle. Those examples, for which only two columns appear under the heading Time-Pressure Cycle were the process of this invention. As. to such examples, the remaining time in the cycle was devoted to the reduction of pressure to zero at the end of the cycle. Those examples for which four columns appear were pressed according to four step cycles typical of prior art pressing operations. 7

Representative small samples were taken from'various portions of the boards and tested, and the test results for. each sample are based on the average results of the tests of several small samples from several difierent boards.

, Since the properties of hardboard vary widely dependpressed according to ing upon a great many factors, including variations in the quality of the wood and ambient conditions at the time of manufacture, the examples are arranged in test pairs for purposes of comparison. Each odd numbered example and its immediately following even numbered example constitute a test pair, i.e., Examples 1 and 2, 3 and 4, 5 and 6, etc., respectively, are test pairs. Each pair was selected from the same lot of boards which were made and tested at the same time and under the same conditions other than as specified herein. Although comparison between all of the examples is not precluded, the most Table of Examples Time-Pressure Cycle, Sec. 1 Pressure (p.s.i.) Fiber Moisture Properties at 1.00 Specific Gravity Content, Percent No. of Total Example Boards Pressing N0. Tested Time Modulus Water Thick Longitu- (Min Initial High Intermediate Coarse Fine of Abs, Swell, dinal Ex- Pressure Pressure Rupture, Percent Percent pansion, p.s.i. Percent 10 2 10/900 40/200 8.6 10.5 6, 200 22 14 0. 39 13 2 10/900 40/400 8. 2 l1. 7 5, 900 22 14 0. 34 3 2 10/800 40/200 7. 9 12. 9 6, 500 19 12 0. 42 3 2 10/850 40/250 9. 4 10.6 6, 200 21 13 0. 45 2 2 10/750 40/150 8. 7 10. 6 7, 600 17 11 0.33 4 2 10/700 40/200 8. 6 10.3 7. 600 17 11 0.32 3 3 10/900 80-100/200 8. 7 l0. 0 5, 000 21 10 0. 34 6 3 10/800 60/200 vent 10/800 7. 8 10. 3 5, 400 23 14 0. 41 6 2 10/850 40/200 7. 7 8. 7 6, 200 25 13 0. 46 6 2 10/750 /200 vent 5/750 8. 2 9. 4 5, 900 28 16 0. 53 9 2 10/850 40/200 8.3 9.8 6,200 22 12 r 0.36 4 2 10/800 30/200 vent 5/800 1.0 13.0 6, 300 25 15 0. 45 4 5 7-30/750 Balance/100 l2 6, 700 19 10 0. 41 11 V 5 5-10/850 Vent 7-15/1000 Balance/100 11 5, 000 23 14 0. 57 5 5 5-10/750 Balance/100 11 5, 900 31 12 0. 47 3 5 5-10/850 Vent 5/1000 Balance/100 12 5, 400 35 16 0. 54 5 5 7/700-1, 000 Balance/100 13 6,400 38 15 0. 56 4 5 4/700-1, 000 Vent 5/1000 Balance/100 12 5, 500 v 44 1S 0. 69 4 2 10/750 30/200 Vent 5/750 1.6 6,100 26 15 0.44 5 2 10/750 30/200 1/50 5/700 9. 4 5, 800 28 16 0. 42 3 2 10/900 40/200 8.0 6, 600 26 14 0. 41 3 1 10/900 40/225 8. 5 6, 800 26 15 0. 36

In each of the examples, several test boards were prepared in a single run from wood fibers comprising 45% hickory, 45% oak, and 10% miscellaneous hardwoods. Each of the sample boards was madefrom mats containing 2.5% resin binder except Examples 1,12 21 and 22, The mats from which each of the sample boards was made contained 2.5%.wax, except Examples 15 through 18, in-

clusive, which contained 1% Wax. The sample boards for which no fine fiber moisture contents are given are single layer boards. All'percentages areby weight of The sample boards were pressed in a hydraulic press between. a pair of imperforate caul plates to an average thickness of about 0.135 inch. Each board was pressed at a temperature of 465 F. exceptExamples 13 to 18, inclusive, which were pressed at 430 F. In each case,

accurate comparison of specific properties is obtained with reference to boards of the same pair. 7 By comparing Examples land 2, 3 and 4, 5 and may be produced by various pressing cycles and particularly various intermediate pressures within the specified 7 limits. 7

In Examples 8, 10, 12, 19 and 20, the sample boards were vented to atmosphere or, in the case 'of Example 20,

reduced to a figure substantially lower. than fp'.s.i. after the intermediate pressure-had been maintained for p common expedient usedby prior art processes but has 6, and 21 and 22, it is readily perceived that good quality boards not been found necessary in the process of this invention. By comparing Examples 7 and 8, 9 and 10, and 11 and 12, it may be seen that the Venting during the intermediate pressure stage in each case resulted in increases in the Water absorption and the amount of expansion both in thickness and in length due to water absorption. By comparing Examples 13 and 14, 15 and 16, and 17 and 18, it may be seen that venting of the mat immediately after application of initial high pressure results in loss of strength, increased moisture absorption, and increased expansion due to moisture absorption. it should be noted that the Examples 15 through 18 contained only 1% wax which accounts for the higher Water absorption than those which contained 2.5% Wax.

Significantly, the boards of Examples 1 through 7, 9, 11, 13, 15, 17, 21 and 22 were characterized by finetextured surfaces which were smooth, hard, and abrasive resistant Without blisters, flow marks or other blemishes. The surface characteristics of the boards of Examples 8, 10, 12, 14, 16 and 18 through 20 were uniformly inferior to those of the other samples.

It will be understood that many variations may be made in the process within the scope of the invention as defined by the appended claims.

What is claimed is:

1. A method of producing a board which comprises forming lignocellulose particles into a compact mat containing from about 6% to about 16% of water and from about 0.5% to about of a resinous binder, said percentages being based on the dry weight of said particles, positioning said mat between a pair of smooth, imperforate surfaces, rapidly subjecting the faces of said mat to a temperature of from about 380 F. to about 520 F. and to an initial high pressure greater than the pressure of saturated steam at the operating temperature and in the range of from about 600 p.s.i. to about 1200 p.s.i., maintaining said initial high pressure until the mat is compressed to a desired density, releasing said initial high pressure at a relatively rapid rate to an intermediate pres sure substantially below the pressure of saturated steam at the operating temperature but not less than 100 p.s.i. to permit the escape of steam from the mat with an audible squeal, maintaining said intermediate pressure until said squealing substantially ceases, and thereafter reducing the pressure applied to said mat to zero to pro duce a board characterized by high strength, high resistance to moisture absorption and hard, smooth, abrasive-resistant surfaces substantially undamaged by escaping steam.

2. A method of producing a board which comprises forming lignocellulose particles into a compact mat containing from about 8% to about 12% of water, from about 0.5% to about 10% of a resinous binder, and from about 1% to about 3.5% of a water repellent, said percentages being based on the dry weight of said particles, positioning said mat between a pair of smooth, imperiorate surfaces, rapidly subjecting the faces of said mat to lit a temperature of from about 400 F. to about 500 F. and to an initial high pressure greater than the pressure of saturated steam at the operating temperature and in the range of from about 750 p.s.i. to about 1000 p.s.i., maintaining said initial high pressure until the mat is compressed to a desired density, releasing said initial high pressure at a relatively rapid rate to an intermediate pressure substantially below the pressure of saturated steam at the operating temperature but not less than p.s.i. to permit the escape of steam from the mat with an audible squeal, maintaining said intermediate pressure until said squealing substantially ceases, and thereafter reducing the pressure applied to said mat to zero to produce a board characterized by high strength, high resistance to moisture absorption and hard, smooth, abrasive-resistant surfaces substantially undamaged by escaping steam.

3. A method of producing a board which comprises forming lignocellulose particles into a compact mat containing from about 8% to about 12% of water, from about 0.5 to about 10% of a resinous binder, and from about 1% to about 3.5% of a water repellent, said percentages being based on the dry Weight of said particles, positioning said mat between a pair of smooth, imperferate surfaces, rapidly subjecting the faces of said mat to a temperature of from about 400 F. to about 500 F. and to an initial high pressure greater than the pressure of saturated steam at the operating temperature and in the range of from about 750 p.s.i. to about 1000 p.s.i., maintaining said initial high pressure until the mat is compressed to a desired density, releasing said initial high pressure at a relatively rapid rate to an intermediate pressure in the range of from at least about 100 p.s.i. to a pressure not in excess of about 100 p.s.i. below the pressure of saturated steam at the operating temperature to permit the escape of steam from the mat with an audible squeal, but not more than about 500 p.s.i., maintaining said intermediate pressure until said squealing substantially ceases, and thereafter reducing the pressure applied to said mat to zero to produce a board characterized by high strength, high resistance to moisture absorption and hard, smooth, abrasive-resistant surfaces substantially undamaged by escaping steam.

4. A method as recited in claim 1 wherein the specific gravity of the finished board is from about 0.8 to about 1.2.

5. A method as recited in claim 2 wherein the specific gravity of the finished board is from about 0.8 to about 1.2.

6. A method as recited in claim 3 wherein the specific gravity of the finished board is from about 0.8 to about 1.2.

References Cited in the file of this patent UNITED STATES PATENTS 2,379,163 Landon June 26, 1945 2,682,083 Patton June 29, 1954 FOREIGN PATENTS 446,510 Canada Apr. 29, 1958 

1. A METHOD OF PRODUCING A BOARD WHICH COMPRISES DORMING LIGNOCELLULOSE PARTICLES INTO A COMPACT MAT CONTAINING FROM ABOUT 6% TO ABOUT 16% OF WATER AND FROM ABOUT 0.5% TO ABOUT 10% OF A RESINOUS BINDER, SAID PERCENTAGES BEING BASED ON THE DRY WEIGHT OF SAID PARTICLES, POSITIONING SAID MAT BETWEEN A PAIR OF SMOOTH, IMPERFORATE SURFACES, RAPIDLY SUBJECTING THE FACES OF SAID MAT OF A TEMPERATURE OF FROM ABOUT 380* F. TO ABOUT 520* F. AND TO AN INITIAL HIGH PRESSURE GREATER THAN THE PRESSURE OF SATURATED STREAM AT THE OPERATING TEMPERATURE AND IN THE RANGE OF FROM AOUT 600 P.S.I. TO ABOUT 1200 P.S.I., MAINTAINING SAID INITIAL HIGH PRESSURE UNTIL THE MAT IS COMPRESSED TO A DESIRED DENSITY, RELEASING SAID INITIAL HIGH PRESSURE AT A RELATIVELY RAPID RATE TO AN INTERMEDIATE PRESSURE SUBSTANTIALLY BELOW THE PRESSURE OF SATURATED STEAM AT THE OPERATING TEMPERATURE BUT NOT LESS THAN 100 P.S.I. TO PERMIT THE ESCAPE OF STEAM FROM THE MAT WITH AN AUDIBLE SQUEAL, MAINTAINING SAID INTERMEDIATE PRESSURE UNTIL SAID SQUEALING SUBSTANTIALLY CEASES, AND THEREAFTER REDUCING THE PRESSURE APPLIED TO SAID MAT TO ZERO TO PRODUCE A BOARD CHARACTERIZED BY HIGH STRENGTH, HIGH RESISTANCE TO MOISTURE ABSORPTION AND HARD, SMOOTH, ABRASIVE-RESISTANT SURFACES SUBSTANTIALLY UNDAMAGED BY ESCAPING STEAM. 